Trimerization and orbital ordering in Ba1-xSrx V13 O18

We studied the physical properties of Ba1-xSrxV13O18 single crystals, which exhibit three different phases caused by the orbital and charge degrees of freedom in V t2g states. We found from the magnetic susceptibility and the NMR measurement that the low-temperature phase of this series of compounds is characterized by the spin-singlet state. We also found from the electrical resistivity and optical conductivity spectra that these compounds become heavily anisotropic in the low-temperature phase with V trimerization, indicating that the orbital ordering of V t2g states causes the anisotropy of the electronic structure of this series of compounds.